The invention relates to a method for the thermal treatment of mealy raw materials, in particular in the manufacturing of cement clinkers from raw meal, whereby the raw meal is treated thermally in a burning process by means of pre-heating, calcination, sintering and cooling, and the exhaust gas flow from the sintering stage (rotary kiln) and an outgoing air stream (tertiary air) of the cooling stage (clinker cooler) are used in the calcination stage, supplied with fuel, for the calcination of the raw meal, whereby the gas-solid material suspension is diverted in the calcination stage, and is introduced into the lowest cyclone of the cyclone suspension gas pre-heater system in order to separate the calcinated raw meal from the gas stream. In addition, the invention relates to an apparatus for the execution of the method.
In installations for the manufacture of cement clinkers from cement raw meal, in order to avoid rotary tubular kilns that are uneconomically long and/or large in diameter, and in order to keep the specific heat requirement of the cement clinker manufacturing process low, it is known to connect a calcinator or, respectively, a calcination stage, upstream from the rotary tubular kiln, seen from the material flow side, the calcinator or, respectively, calcination stage, being equipped with a second firing (in addition to the firing in the rotary tubular kiln). A calcinator is known in which an ascending pipeline branch that is supplied with fuel and that conducts the gas-solid material suspension is diverted 180xc2x0 into a descending pipeline branch (e.g. EP-B-0 222 044; also EP-B-0 526 770). The fuel, of whatever sort (for example, coal dust), that is introduced into the calcination stage is combusted as completely as possible with hot outgoing air (called tertiary air) coming from the clinker cooler; that is, it is combusted with an excess of oxygen, in particular in the tertiary air line itself. The combustion heat that arises is transferred immediately to the raw meal, and is used for the calcination of the raw meal before introduction into the rotary tubular kiln, whereby the temperature does not increase significantly beyond the dissociation temperature of the de-carbonatization reaction. Today, efforts are being made to burn up to about 65% of the total fuel required for a cement clinker production line in the calcinator connected upstream from the rotary tubular kiln, and to burn only the remaining 35% of the fuel in the rotary tubular kiln itself. That is, in modem installations the fuel is thus used predominantly in the calcinator, because the specific heat requirement or, respectively, heat consumption in the calcinator, at the level of approx. 550 kcal per kg of de-acidified raw meal for the execution of the endothermic calcination (de-acidification) reaction, is higher than the heat requirement still occurring in the rotary tubular kiln connected downstream. What is concerned is thus a calcination that is as complete as possible of the raw meal in the calcination stage connected upstream from the rotary tubular kiln.
In the calcinator of the two references cited above, the rotary kiln exhaust gas is combined with the tertiary air coming from the clinker cooler. Before the combination, in the rotary kiln ascending exhaust gas pipeline, fuel is burned sub-stoichiometrically, i.e., with a deficiency of oxygen, for the purpose of creating a reduction zone that contains CO, or, respectively, a strand of CO for the reduction of the harmful substance NOx, which is formed in particular by the high-temperature combustion in the rotary tubular kiln (thermal NOx), while in the adjacent tertiary air duct fuel is burned super stoichiometrically, i.e., with an excess of oxygen. Seen opposite the flow of the suspension, the CO not consumed in the NOx reduction zone is then burned using excess oxygen from the tertiary air duct, whereby the burning of this remainder is favored by the 180xc2x0 angle in the pipeline, or, respectively, in the known specific construction, by a turbulence chamber or, respectively, mixing chamber arranged in the area of the pipeline angle.
Using the technologies described above, the NOx emissions of cement clinker burning installations can be reduced very significantly. However, there are cases of application in which a further reduction of NOx is desirable, in particular also a reduction of the fuel NOx that arises in the calcination stage during the combustion of the fuel in the tertiary air duct, formed by the nitrogen that is chemically bound in the fuel.
The invention is based on the object of creating, in cement clinker production lines of the type named above, a calcinator that is connected upstream from the rotary tubular kiln and is equipped with two firings, which enables on the one hand a high-grade calcinated raw meal and on the other hand an exhaust gas having an even further reduced NOx content, as well as a burning that is as complete as possible of remainders from CO gas strands, and also of other fuel components.
It is characteristic for the inventive calcination stage or, respectively, the calcinator that both the fuel introduced into the rotary kiln exhaust gas line and also the fuel introduced into the cooler outgoing air line (tertiary air duct) is burned sub-stoichiometrically, i.e. with a deficiency of oxygen, for the purpose of forming a respective CO-containing reduction zone (CO strand) for NOx reduction both in the rotary kiln exhaust gas line and in the tertiary air duct. In other words, both in the rotary kiln exhaust gas duct and in the tertiary air duct there is at least one combustion point such that there is a strong excess of CO in the calcination stage, whereby the NOx content in the calcinator exhaust gas, as well as in the exhaust gas of the overall cement clinker production line, can be further lowered. In order to avoid emissions of CO here, in the calcinatorxe2x80x94regarded downstream from the flow of the suspensionxe2x80x94in the calcination stage itself, combustion air is introduced into the gas-solid material suspension for the burning of the remainder of the CO strands. According to a particular feature of the invention, this combustion air for the afterburning of the CO strands coming from the reduction zone, as well as, if warranted, other fuel components, is branched off from the tertiary air, which supplies the combustion point located in the calcination stage in the tertiary air duct with oxygen. The quantity of branched-off tertiary air can be approximately 10 to 50% of the overall tertiary air supplied to the calcination stage.
According to a particular feature of the invention, the air ratio xcex for the sub-stoichiometric fuel combustion in the rotary kiln exhaust gas duct is smaller than in the tertiary air duct; i.e., in the first-named combustion point it can lie in a range from approximately 0.1 to 0.7, and in the second-named combustion point it can lie in a range from approximately 0.5 to 1.0.
In a cement clinker production line, the invention enables an effective further lowering of the NOx contained in the exhaust gas from the firing of the rotary tubular kiln (here mainly thermal NOx) and from the firing of the calcinator (here mainly fuel NOx), with simultaneous creation of high-quality (e.g. greater than 90%) calcinated cement raw meal before entry into the rotary tubular kiln.
The invention, and its additional features and advantages, are explained in more detail on the basis of the exemplary embodiment shown schematically in the drawing.